347. 前 K 个高频元素

class Solution 
{
public:
    vector<int> topKFrequent(vector<int>& nums, int k) 
    {
        vector<int> res;
        unordered_map<int,int> hash;
        for(auto a : nums) hash[a] ++;
        priority_queue<pair<int,int>> pq;
        for(auto a : hash) pq.push({a.second,a.first});
        while(k --)
        {
            if(!pq.empty())
            {
                res.push_back(pq.top().second);
                pq.pop();
            }
        }
        return res;
    }
};

 

class Solution 
{
public:
    //对出现的次数进行排序，且从大到小进行排序
    //你可以按任意顺序返回答案
    int partition(vector<pair<int,int>> &a,int start, int end)
    {
        //int index = start + rand() % (end - start + 1);
        //swap(a[index],a[end]); //随机化

        int small = start - 1;
        for(int i = start;i < end;i ++)
            if(a[i].second > a[end].second)     
                swap(a[i],a[++small]);
        swap(a[++small],a[end]);
        return small;
    }

    void select(vector<pair<int,int>> &a,int l,int r,int k)
    {
        if(l >= r) return;
        int j = partition(a,l,r);
        int k1 = k - 1;
        if(k1 == j) return; // j刚好等于k1 
        else if(k1 < j) select(a,l,j - 1,k);
        else select(a,j + 1,r,k);
    }

    vector<int> topKFrequent(vector<int>& nums, int k) 
    {
        unordered_map<int,int> ma;
        for(int i : nums) ma[i] ++;
        vector<pair<int,int>> ve(ma.begin(),ma.end());
        vector<int> res;
        select(ve,0,ve.size() - 1,k);
        for(int i = 0;i < k;i ++) res.push_back(ve[i].first);
        return res;
    }
};